M. Járó - L. Költő szerk.: Archaeometrical research in Hungary (Budapest, 1988)
Prospecting - KISS János, PATTANTYUS-Á. Miklós, SELMECZI László: Geophysical research at a Jazygian' settlement, Négyszállás
0 lOm Fig. 2 Apparent resistivity map (see caption to Fig. 1 for measuring data). The maxima provide information on the structure of the settlement. The strikingly high resistivity zone of the NW part indicates the place of a 14th century house, according to the excavation having been performed since then (place of the excavation is marked (see Photo 1). Experimental electromagnetic measurements The well-known disadvantage of direct current resistivity measurements (placing the electrodes into the ground) can be avoided by employing the electromagnetic method. Such an inductive soil-conductivity meter (for shallow depths) is the EM-31 (Geonics) instrument. The EM— 31 equipment enables rapid and convenient work; its data logger and computer processin g system (developedin ELGI) ensure accurate processing (computer map drawing, filtering, representation with site plan). If the measurement is performed with horizontal coil position at a height of 1 m, 80 percent of the eddy currents induced in the soil flows in the upper 5 m. In the case of vertical coil position this depth decreases to 3 m.The measurement height has a significant influence on the current distribution, therefore on the detection of the features being at different depths too. In this way, on the basis of the depth of the archaeological objects, the suitable instrument position can optimally be chosen. This equipment, just like the electromagnetic method itself, is sensitive to the conductive bodies in a higher resistivity environment (in other words, to the contrast decreasing in resistivity). In the case of most archaeological objects, however, an increasing resistivity contrast must be counted on, i. e. the resistivity of the objects is higher than that of their environment. Therefore , the expectable differences in conductivity may be very slight. The resolution can be increased by means of the vertical gradient calculated from measurements of several instrument positions and heights. The information content of the gradient mainly derives from the depth of about 0.5 — 1 m in the case of horizontal coils (for 1 m and 0 m instrument heights) — according to mathematical modelling. At Jászdózsa, on the NW part of one of the areas measured by direct current resistivity measurement (Fig. 2), experimental electromagnetic conductivity measurements were performed ir an area of 10X20 m, in a grid corresponding to the resistivity measurements. This area includes the resistivity maximum, at which the excavation was located.
